Modeling structure effects on aggregation kinetics in colloidal dispersions

The time evolution of the cluster-mass distribution (CMD) during colloidal aggregation can be modeled using population balance equations, given that the matrix of the aggregation rate constant (kernel) is known. Although numerous aggregation kernels have been proposed, their validity is still a major open problem, particularly when the role of the internal structure of the aggregates is referred to. A procedure is presented for the discrimination among possible kernel expressions including the structure effect. For aggregation processes in the submicron range, information about size and structure of aggregates can be obtained by dynamic and static light-scattering measurements, for example, in terms of the average hydrodynamic (Rh) and gyration (Rg) radii. These quantities can also be calculated from the cluster-mass distribution when accounting for the aggregate structure by the fractal concept and for the angular and rotational diffusion dependence of Rh. Since Rh and Rg represent different averages of the CMD, their simultaneous fitting is a severe test for a given kernel due to its inclusion of information on the average and width of the distribution. This procedure allows differentiation among several types of kernels proposed in the literature for DLCA and RLCA based on their ability to describe experimental data.